Crib mattresses are made in a standard size to fit the standard crib frame. A mattress that is too small or a frame that is too large will pose a safety hazard, so it is important to check that the mattress is the right size for your crib.
The standard crib mattress measures at least 51 5/8 inches long by 27 1/4 inches wide. This will fit in the standard crib measuring 52 inches by 28 inches.
Most crib mattresses are between 4 and 6 inches deep. Mattresses with a depth of over 6 inches can pose a safety hazard because the crib rails may not be high enough.
If you have an older crib or a specialty crib, it may require a different size mattress. Check the packaging for instructions, or contact the manufacturer.
To make sure the crib mattress fits snugly enough, put your finger between the mattress and the slats of the crib. If you can then fit one more finger in, the mattress is too small, and the baby may get trapped between the mattress and the crib.
When you purchase a crib and a mattress, return the product registration cards so you will be notified if there is a recall, as was the case in 2006-2008 with some Simmons crib mattresses.